1. Field of the Invention
The present invention is related to a method for measuring mechanical vibrations of an object. Furthermore, the invention is related to a system for measuring such vibrations of an object.
2. Description of the Prior Art
The frequency spectrum of a signal representative of the vibrations of, e.g., an axial compressor or any other rotational industrial or aeronautical machine shows generally two rather different domains:
1) At low frequencies, for example in a frequency range lower than 1000 or 2000 Hertz, the vibration signal has a rather low level in terms of acceleration and is for example inferior to 1 m/s2.
In the fields of aeronautics and industry, a low frequency vibration signal is transformed or measured in terms of vibration velocity which is representative of the vibration energy, often destructive, which acts on a rotary machine.
The ranges of low frequencies and the levels of acceptable vibration signals in these ranges are broadly standardized in function of the machine, for example:
according to ISO standard No. 2954/VDI2056, between 10 and 1000 Hertz;
according to API standard No. 670, between 10 and 2500 Hertz, etc.
At low frequencies, the level of a vibration signal at the fundamental rotational frequency constitutes a particular interest in counterbalancing principal rotors (for example in the case of a blower, a compressor and a turbine, a propeller, etc.). The level of the vibration signal at harmonics at low frequencies has also a certain interest in certain analyses.
2) At medium and high frequencies (for example at frequencies higher than 1000 Hertz), lines corresponding for example to gearing vibrations, to multiples and modulation of such lines, lines corresponding to the passage of turbine blades, to signatures of rolling bearings, etc., can be found in the frequency spectrum of a signal representative of the vibrations of an object.
In the field of industry, a vibration signal of medium and high frequencies is measured in terms of the vibration acceleration, and its level is relatively high and often greater than to 10 m/s2.
In the fields of civil and, above all, military aeronautics, the vibration acceleration at high frequencies may attain 10,000 m/s2; such vibrations can provoke saturation of the first amplification stage in the measurement chain.
The above-indicated strongly pronounced differences between the features of a low frequency vibration signal and the features of the same signal at medium and high frequencies result in the necessity, for certain applications as, for example, the measurement of vibrations acting upon a turbo-propeller group of an aircraft, to install two different measurement chains, namely a first measurement chain for monitoring the low frequency component of the vibrating signal corresponding to the vibrations related to the rotation of the propeller, and a second measurement chain for monitoring, in the field of high frequencies, the component of the vibrating signal corresponding to the vibrations of the reduction gear. Such a solution has certain disadvantages, namely not only the price increase of the measurement system but also a lower reliability of such a system, since it is more complex and, as a consequence, the probability of defects is higher.
Therefore, the aim of the present invention is to provide a method and a system for measuring mechanical vibrations which allows to overcome the above-discussed drawbacks.
According to a first aspect of the invention this aim is attained with a method for measuring the mechanical vibrations of an object, comprising
electronically processing an input signal, representative of an acceleration related to a mechanical vibration of said object and having a frequency spectrum comprising a low-frequency band situated below a transition frequency and a high frequency band situated above said transition frequency,
said processing being carried out by means of an electronic circuit which is adapted to provide an output signal which
within said low-frequency band corresponds to the mathematical integral over the time of said input signal, i.e. to the vibration velocity of said object, and
within said high-frequency band corresponds to the input signal, i.e. to the vibration acceleration of said object.
According to a second aspect of the invention, the above mentioned aim is attained with a method for measuring the mechanical vibrations of an object, comprising
electronically processing an input signal, representative of a velocity related to a mechanical vibration of said object and having a frequency spectrum comprising a low-frequency band situated below a transition frequency and a high frequency band situated above said transition frequency,
said processing being carried out by means of an electronic circuit which is adapted to provide an output signal which
within said low-frequency band corresponds to the mathematical integral over the time of said input signal, i.e. to the vibration displacement of said object, and
within said high-frequency band corresponds to the input signal, i.e. to the vibration velocity of said object.
According to a third aspect of the invention, the above mentioned aim is attained with a system for measuring mechanical vibrations of an object, comprising
(a) a transducer mounted on said object and being capable of providing at its output an input signal which is representative of an acceleration related to a mechanical vibration of said object, said input signal comprising a low-frequency band situated below a transition frequency and a high frequency band situated above said transition frequency, and
(b) an electronic circuit having an input port, connected to the output of said transducer, and an output port, said circuit being adapted to process said input signal to yield an output signal which within said low-frequency band corresponds to the mathematical integral over the time of said input signal, and which within said high-frequency band corresponds to said input signal.
According to a fourth aspect of the invention, the above mentioned aim is attained with a system for measuring mechanical vibrations of an object, comprising
(a) a transducer mounted on said object and being capable of providing at its output an input signal which is representative of a velocity related to a mechanical vibration of said object, said input signal comprising a low-frequency band situated below a transition frequency and a high frequency band situated above said transition frequency, and
(b) an electronic circuit having an input port, connected to the output of said transducer, and an output port, said circuit being adapted to process said input signal to yield an output signal which within said low-frequency band corresponds to the mathematical integral over the time of said input signal, and which within said high-frequency band corresponds to said input signal.
The main advantage of the invention is that it allows to obtain, by means of a sole transducer and a sole measurement chain, a single output signal which, in the range of low frequencies, corresponds to the vibration velocity, and in the range of medium and high frequencies, to the vibration acceleration. The information thus obtained on the vibration velocity may be used to quantify the severity of vibrations at low frequencies and to take the necessary measures for the protection of the machine, whereas the information obtained on the vibration acceleration may be used above all for the diagnostic and the health of mechanical components such as bearings, blade arrays and gearings which generate in particular the high frequency vibrations when their mechanical condition degrades.
Furthermore, the invention offers the following advantage:
It permits either to increase the level of the signal at low frequencies where the problem of background noise often limits the behavior of the measurement chains, or to decrease the amplitudes at high frequencies which could provoke the saturation in the following stages of the measurement chain. In both cases, this advantage provides an optimization of the signal dynamics. In a simple example of an industrial turbo-generator, it has been calculated that the necessary dynamic was optimized by about 30 dB. This great improvement reduces the performance requirements downstream the collecting chains and allows for example
to simplify the attenuators or amplifiers at the input port of the signal processing unit, and
to reduce the resolution performance of the analog-digital converters (ADC) in terms of required bits.
The invention thus allows to reach simultaneously two goals, namely, on one hand, an improvement of the dynamics in the processing of the measurement signal, and, on the other hand, the supply of two physical quantities of current use in vibration analysis, namely acceleration and velocity, and this on a single output signal stemming from a sole transducer.
Still another advantage of the invention is the fact that it allows to increase the productivity of measurements which can be carried out by means of a system for measuring mechanical vibrations. In fact, for dynamic reasons during the signal processing, it has been necessary until now to carry out two separated analyses: one within the range of low frequencies, and the other in the range of medium and high frequencies. Each one of these analyses consists in a type FFT (Fast Fourier Transformation) processing which is a long-lasting one for the operators and the processor. Since the present invention allows to modify or adapt the signal dynamics, the analysis of the measurement signal can be carried out in a single run, resulting in a gain of time and in a simplification in the management of the vibration database.
It is possible, through a repeated application of the method according to the invention, to obtain a single output signal carrying information on three physical quantities such as displacement, velocity and acceleration. The applications of this variant are, however, more limited.
Embodiments of the invention will now be described by way of Examples in referring to the attached drawings.